1. Field of Invention
The present invention is directed to a device for comminution of solids. More particularly, the present invention relates to a conically-shaped impact mill.
2. Description of the Prior Art
Devices for providing comminution of particulate solids are well known in the art. Amongst the many different milling devices known in the art grinding mills, ball mills, rod mills, impact mills and jet mills are most often employed. Of these, only jet mills do not rely on the interaction between the particulate solid and another surface to effectuate particle disintegration.
Jet mills effectuate comminution by utilization of a working fluid which is accelerated to high speed using fluid pressure and accelerated venturi nozzles. The particles collide with a target, such as a deflecting surface, or with other moving particles in the chamber, resulting in size reduction. Operating speeds of jet milled particles are generally in the 150 and 300 meters per second range. Jet mills, although effective, cannot control the extent of comminution. This oftentimes results in the production of an excess percentage of undersized particles.
Impact mills, on the other hand, rely on centrifugal force, wherein particle comminution is effected by impact between the circularly accelerated particles, which are constrained to a peripheral space, and a stationary outer circumferential wall. Again, although control of particle size distribution is improved and can be manipulated compared to jet mills, the particle size range of the comminuted product of an impact mill is fixed by the dimensions of the device and other operating parameters.
A major advance in impact mill design is provided by a design of the type disclosed in German Patent Publication 2353907. That impact mill includes a base portion which carries a rotor, mounted in a bearing housing having an upwardly aligned cylindrical wall portion coaxial with the rotational axis, and a mill casing which surrounds the rotor, defining a conical grinding path. The mill of this design includes a downwardly aligned cylindrical collar which may be displaced axially in the cylindrical wall portion and may be adjusted axially to set the grinding gap between the rotor and the grinding path.
An example of such a design is set forth in European Patent 0 787 528. The invention of that patent resides in the capability of dismantling the mill casing from the base portion in a simple manner.
Although impact mills having conical shapes, permitting a downwardly aligned cylindrical collar to be displaced axially so that the grinding gap may be adjusted, represents a major advance in the art, still those designs can be improved by further design improvements that have not heretofore been addressed.
Impact mills, when utilized in the comminution of elastic particles, such as rubber, are usually operated at cryogenic temperatures, utilizing cryogenic fluids, in order to make feasible effective comminution of the otherwise elastic particles. Commonly, cryogenic fluids, such as liquid nitrogen, are utilized to make brittle such elastic solid particles. In view of the fact that the cryogenic temperatures attained by the frozen particles are much lower than the ambient surrounding temperature of the mill, this temperature gradient results in a rapid temperature rise of the particles. As a result, it is apparent that maximum comminution in an impact mill, or any other mill, should begin immediately after particles freezing. However, impact mills, including the conically shaped design discussed supra, initially require the particles to move outwardly toward the periphery before comminution begins. During that period the temperature of the particles is increased, reducing comminution effectiveness.
Another problem associated with comminution mills in general and conical mills of the type described above in particular is the inability to alter the physical configuration of the impact mill to adjust for specific particle size requirements of the various materials.
Three expedients are generally utilized to change the particle size of an elastic solid whose initial size is fixed.
The first expedient employed in changing particle size is changing the feedstock temperature by contact with a cryogenic fluid, e.g. liquid nitrogen, to freeze the elastic solid particles to a crystalline state. The coldest temperature achievable by the particles is limited to the temperature of the cryogenic fluid. A means of controlling particle temperature is to adjust the quantity of cryogenic fluid delivered to the elastic solid particles.
A second expedient of changing product particle size is to alter the peripheral velocity of the rotor. This is usually difficult or impractical given the physical limits of the impact mill design.
A third expedient of altering particle size is to change the grinding gap between the impact elements. Generally, this step requires a revised rotor configuration.
An associated problem, related to alteration of rotor configuration in order to effect changes in desired product particle size, is ease of replacement of worn or damaged portions of the impact mill. As in the case of replacement of parts of any mechanical device, problems are magnified in proportion to the size and complexity of the part being replaced.
Yet another problem associated with impact mills resides in power transmission to effectuate rotation of the rotor. Present designs employ multiple belt or gear power transmission means which are oftentimes accompanied by unacceptable noise levels. A corollary of this problem is that if power transmission speeds are reduced to abate excessive noise, rotor speed is reduced so that comminution results are unacceptable. It is thus apparent that a method of improved power transmission, unaccompanied by unacceptable loud noise, is essential to improved operation of impact mills.